1. Field of the Invention
This invention relates to a print head mounted on a dot matrix printer of the impact type, and more particularly to improvements in such print heads.
2. Discussion of the Prior Art.
A conventional print head of the above mentioned type is depicted, for example, in FIG. 1. The depicted print head may be classified as a release type head comprising a free end 2a of a spring 2 secured at the root thereof between a frame 1a and a yoke 1b which is fixed to one end of an armature 4 which is fixed at the other end thereof to a print wire 3. That is, free end 2a of spring plate 2 is fixed to a holding part 5 formed in one end of the armature usually by brazing. Armature 4 is attracted to core 8a of an electromagnetic device 8, by a permanent magnet 7, against the resilience of spring plate 2 and is released from attraction of permanent magnet 7, by exciting electromagnetic device 8. In this manner, the resilience of spring plate 2 drives print wire 3 selectively for printing action.
When electromagnetic device 8 is not excited, and hence spring plate 2 is bent against the resilience thereof toward core 8a, a compressive stress and a tensile stress are concentrated on the brazed part 6a on the side of core 8a and on the opposite brazed part 6b, respectively. Consequently, brazed part 6b is likely to be fissured by the concentrated tensile stress during repeated printing operation, thereby resulting in faulty connection of spring plate 2 and armature 4. Thus, reliability is adversely affected, and the life of the print head is considerably reduced. Since the surface of free end 2a joined toholding part 5 by brazing is alloyed with a brazing solder and brittle, free end 2a, in particular, is likely to be broken by the tensile stress, which acts directly on the brittle surface of the brazed surface of free end 2a when brazed part 6b is cracked.
Thus, an urgent need exists in the art for a print head which has a greater lifetime, and increased reliability.